Crash tests on several of the GM hybrid cars and their batteries led to fires, U.S. agency says.

For General Motors Co., the Chevrolet Volt may be generating the wrong kind of spark.

Federal officials on Friday launched a formal safety defect investigation into GM's plug-in hybrid vehicle after crash tests on several Volts and their batteries resulted in fires. In one case, a fire that started in one of the test vehicles consumed three others parked nearby.

The National Highway Traffic Safety Administration said it was "concerned" that damage to the Volt's batteries sustained in tests designed to replicate real-world crash scenarios resulted in the vehicles' catching fire.

The agency said it was too soon to say whether the probe would lead to a recall of the vehicle or any of its parts but vowed to "take immediate action" if it found any unreasonable safety risk. Measures would include notifying consumers and ensuring that GM communicates with Volt owners.

The investigation comes as GM has pushed for wider adoption of the plug-in hybrid, which according to the Environmental Protection Agency gets an equivalent of 93 miles per gallon. GM has sold more than 6,000 Volts since the car was rolled out in December 2010. The Volt, which won the Green Car of the Year award at the Los Angeles Auto Show last year, is designed to run off its batteries for about 40 miles. When the batteries run low, a gasoline engine kicks in and functions as a generator, powering electric motors and extending the range of the sedan to more than 300 miles.

Analysts said that the investigation could lead to increased consumer skepticism about electric vehicles, which have been slow to catch on in the mainstream because of concerns about cost-effectiveness, and how far the vehicles can travel before recharging.

"It's a technology-laden car, and there are still issues with batteries," said Dave Sullivan, an analyst at consulting firm AutoPacific, noting that the Volt uses the same type of lithium-ion batteries found in consumer electronics products such as laptops that have been known to overheat. "GM's going to have to reopen their safety book to prove that these vehicles are indeed safe."

GM acknowledged the investigation, saying it was "not unexpected" and that the company had been working with NHTSA to reproduce the problem for months. The company said that Volt owners who have not been involved in a severe accident have nothing to worry about.

"The Volt is a safe car," said Greg Martin, a spokesman for GM. "What occurred was what all parties set out to do, and that was to subject the battery in a laboratory setting to some of the most extreme circumstances to get it to fail."

GM and NHTSA will examine data from the tests to attempt to determine the precise cause of the fires.

The latest tests were part of an initial probe begun after NHTSA found a Volt it had crash-tested in May caught fire three weeks later.

In a series of follow-up tests, NHTSA subjected three additional Volt batteries to a crash simulation last week, intentionally damaging the cars' battery compartment and rupturing their coolant lines. One battery caught fire about a week after the test. A second vehicle's battery "began to smoke and emit sparks" just hours after the impact, the agency said.